The present invention relates to an antenna for use in a mobile radio device etc. and, in particular, to a multi-band antenna which can carry out transmission and reception at a plurality of mutually different frequency bands.
Recently, there have been those regions and countries wherein a plurality of portable telephone systems using different frequency bands are available. For example, in Japan, the PDC system uses 800 MHz and 1.5 GHz bands, while the PHS system uses a 1.9 GHz band. On the other hand, 800 MHz and 1.9 GHz bands are used in U.S.A., while 900 MHz and 1.8 GHz bands are used in Europe.
Following the recent remarkable spread of portable terminals, there has been caused congestion with respect to a certain frequency band. In view of this, there have been such requests that each portable terminal can carry out transmission and reception at a plurality of frequency bands. According to the requests, when a first frequency band allocated initially is crowded or in case of a region where such a first frequency band is not available, transmission and reception can be performed using a second frequency band allocated secondarily.
In general, when a radio device is used at different frequency bands, a plurality of antennas are used. As a typical example of such a radio device, an FM/AM radio set can be cited. In contrast, there has been a trap antenna which is so arranged as to be used at different frequency bands. The trap antennas have been widely used in amateur radio devices as multi-band antennas.
For example, JP-A-5-121924 discloses a conventional trap antenna. The disclosed trap antenna comprises a linear antenna element and a trap circuit having a coil and a capacitor.
However, there has been a problem that the number of parts and the number of manufacturing processes required for the conventional trap antenna are both large. Further, when the conventional trap antenna is externally attached to the radio device in an exposed fashion, it is defective in strength such that the coil and the capacitor tend to be damaged when subjected to a shock etc. This raises a serious problem with respect to a portable terminal which is supposed to be carried. Further, since the conventional trap antenna can not be drawn out to the exterior of the radio device and has only a small gain, a problem may be raised that, in particular, transmission characteristics can not be ensured upon transmission from the radio device. Moreover, since a structure of the conventional trap antenna is complicated, the size reduction thereof is difficult to achieve. There have been further problems that dispersion in resonance frequency of the conventional trap antennas is increased depending on manufacturing processes, the productivity thereof is low and it is relatively heavy.
Therefore, the conventional trap antenna can not be said to be suitable for use in a portable terminal for the portable telephone system.